Stack Questions at Zeta: What to Expect

Stack questions appear in about 14% of Zeta's technical interviews (5 out of 35 problems). This frequency is significant enough that neglecting stack fundamentals can create a major gap in your preparation. At Zeta, a financial technology company, stack problems often model real-world scenarios involving ordered data processing, transaction validation, or parsing nested structures—core operations in payment systems and ledger management. Mastering stacks demonstrates your ability to handle sequential data with LIFO logic, a skill directly applicable to their domain.

What to Expect — Types of Problems

Zeta’s stack problems typically fall into three categories. First, parsing and validation questions, like checking for balanced parentheses or valid HTML tags, test your ability to track opening and closing symbols. Second, monotonic stack problems, which involve finding the next greater or smaller element in an array, are common for assessing optimization skills on sequential data. Third, you may encounter stack transformations, where you must simulate a sequence of push/pop operations or use a stack to implement a queue. The problems are designed to be solved optimally with a single stack, often requiring O(n) time complexity.

How to Prepare — Study Tips with One Code Example

Focus on understanding the core pattern: use a stack to temporarily hold items while you compare them to incoming elements, processing them when a condition is met. Always clarify the problem, identify the LIFO need, and walk through an example manually before coding. Practice writing clean, iterative solutions.

A key pattern is the Next Greater Element problem. You traverse an array, using a stack to hold indices of elements for which we haven't yet found a greater element. When you find a greater value, you resolve the indices on the stack.

def nextGreaterElement(nums):
    result = [-1] * len(nums)
    stack = []  # stores indices

    for i, num in enumerate(nums):
        while stack and nums[stack[-1]] < num:
            idx = stack.pop()
            result[idx] = num
        stack.append(i)
    return result

Recommended Practice Order

Start with foundational stack operations: implement a stack using an array or linked list. Then, solve classic validation problems like valid parentheses. Move on to monotonic stack patterns, such as next greater element and daily temperatures. Finally, tackle more complex applications, including stack-based calculator (evaluate expression) and histogram area problems. This progression builds from mechanics to applied problem-solving.

Practice Stack at Zeta